Natural gas production may be complicated by the presence of certain heavy hydrocarbons in the subterranean formation in which the gas is found. Under conditions prevailing in the subterranean reservoirs, the heavy hydrocarbons may be partially dissolved in the compressed gas or finely divided in a liquid phase. The decrease in temperature and pressure attendant to the upward flow of gas as it is produced to the surface result in the separation of solid hydrocarbonaceous material from the gas. Such solid hydrocarbons may form in certain critical places such as on the interior wall of the production string, thus restricting or actually plugging the flow passageway.
Many hydrocarbonaceous mineral streams contain some small proportion of these diamondoid compounds. These high boiling, saturated, three-dimensional polycyclic organics are illustrated by adamantane, diamantane, triamantane and various side chain substituted homologues, particularly the methyl derivatives. Diamondoid compounds have high melting points and high vapor pressures for their molecular weights and have recently been found to cause problems during production and refining of hydrocarbonaceous minerals, particularly natural gas, by condensing out and solidifying, thereby clogging pipes and other pieces of equipment. For a survey of the chemistry of diamondoid compounds, see Fort, Jr., Raymond C., The Chemistry of Diamond Molecules, Marcel Dekker, 1976.
In recent times, new sources of hydrocarbon minerals have been brought into production which, for some unknown reason, have substantially larger concentrations of diamondoid compounds. Whereas in the past, the amount of diamondoid compounds has been too small to cause operational problems such as production cooler plugging, now these compounds represent both a larger problem and a larger opportunity. The presence of diamondoid compounds in natural gas has been found to cause plugging in the process equipment requiring costly maintenance downtime to remove. On the other hand, these very compounds which can deleteriously affect the profitability of natural gas production are themselves valuable products.
Various processes have been developed to prevent the formation of such precipitates or to remove them once they have formed. These include mechanical removal of the deposits and the batchwise or continuous injection of a suitable solvent. Recovery of one such class of heavy hydrocarbons, i.e. diamondoid materials, from natural gas is detailed in commonly assigned allowed U.S. patent application Ser. No. 405,119, filed Sep. 7, 1989, which is a continuation of Ser. No. 358,758, filed May 26, 1989, now abandoned, as well as allowed U.S. patent application Ser. Nos. 358,759; 358,760; and 358,761, all filed May 26, 1989. The text of these allowed U.S. patent applications is incorporated herein by reference.
Research efforts have more recently been focused on separating diamondoid compounds from the liquid solvent stream described, for example, in the above cited U.S. patent application Ser. No. 405,119. The diamondoid and solvent components have proven difficult to separate via conventional multistage distillation due at least in part to the overlapping boiling ranges of the preferred solvents and the commonly occurring diamondoid compounds. Further, the diamondoid compounds have been found to deposit precipitate in the overhead condenser circuit of a solvent distillation apparatus. Developing the commercial potential of these valuable components is then predicated upon the discovery of an economical method for separating diamondoids from the solvent.
Many compounds are known to form azeotropes, liquid mixtures of two or more substances which behave as a single substance in that the vapor produced by partial evaporation of liquid has the same composition as the liquid. Azeotropic distillation, then, is a type of fractionation in which a substance is added to the mixture to be separated in order to form an azeotropic mixture with one or more of the components of the original mixture. The azeotrope or azeotropes thus formed will have boiling points different from the boiling points of the original mixture, thus facilitating separation. See Sax and Lewis, Hawley's Condensed Chemical Dictionary, 109 (11th ed., 1987) and 3 Kirk-Othmer Encyclopaedia of Chemical Technology 352 (3rd ed., 1978).
Whether an azeotrope will form at all, as well as whether the resulting azeotropic mixture will boil at a temperature above or below that of the original mixture, cannot readily be predicted. Developing an azeotropic fractionation process which would be practical on an industrial scale presents a still greater challenge because the selected co-distillate must not only form an azeotrope which is readily separable from the original mixture, but must also be available at a reasonable cost.